[PATCH v2 1/1] qemu-img: do not erase destination file in qemu-img dd command

[Mike Maslenkin <mike.maslenkin@gmail.com>]

Add a check that destination file exists and do not call bdrv_create for
this case.

Currently `qemu-img dd` command destroys content of destination file.
Effectively this means that parameters (geometry) of destination image
file are changing. This can be undesirable behavior for user especially
if format of destination image does not support resizing.

Steps to reproduce:
  1. Create empty disk image with some non default size.
       `qemu-img  create -f qcow2 $DEST_IMG 3T`
     Remember that `qemu-img info $DEST_IMG` returns:
       virtual size: 3 TiB (3298534883328 bytes)
       disk size: 240 KiB
       cluster_size: 65536
  2. Run `qemu-img dd -O qcow2 of=$DEST_IMG if=$SRC_IMG bs=1M count=100`
  3. Check `qemu-img info $DEST_IMG` output:
       virtual size: 100 MiB (104857600 bytes)
       disk size: 112 MiB
       cluster_size: 65536

Parameters of $DEST_IMG were changed. Actually `qemu-img dd` has created
a new disk based on current default geometry for particular format.
For example for "parallels" format default BAT for 256GB disk is written
to empty file prior writing disk image data.

With this patch virtual disk metadata and geometry of a destination image
are preserved. As another visible change of `qemu-img dd` behavior is that
if destination image is less than source it can finish with error (similar
to "dd" utility):
  qemu-img: error while writing to output image file: Input/output error

Signed-off-by: Mike Maslenkin <mike.maslenkin@gmail.com>
---
  diff from v1: removed additional fprintf call leaved in patch by accident
---
 qemu-img.c | 17 ++++++++++-------
 1 file changed, 10 insertions(+), 7 deletions(-)

diff --git a/qemu-img.c b/qemu-img.c
index a48edb71015c..1a83c14212fb 100644
--- a/qemu-img.c
+++ b/qemu-img.c
@@ -5150,13 +5150,15 @@ static int img_dd(int argc, char **argv)
                             size - in.bsz * in.offset, &error_abort);
     }
 
-    ret = bdrv_create(drv, out.filename, opts, &local_err);
-    if (ret < 0) {
-        error_reportf_err(local_err,
-                          "%s: error while creating output image: ",
-                          out.filename);
-        ret = -1;
-        goto out;
+    if (!g_file_test(out.filename, G_FILE_TEST_EXISTS)) {
+        ret = bdrv_create(drv, out.filename, opts, &local_err);
+        if (ret < 0) {
+            error_reportf_err(local_err,
+                               "%s: error while creating output image: ",
+                               out.filename);
+            ret = -1;
+            goto out;
+        }
     }
 
     /* TODO, we can't honour --image-opts for the target,
-- 
2.32.0 (Apple Git-132)